One of conventional differential gear cases (which will be called “differential case” in the following) is a container housing (see the case in Patent Document 1, for example) that contains therein a differential gear and has on an outer surface thereof an annular flange that extends around an axis of rotation of the container housing and has, on one of two areas of the outer surface of the container housing that are bordered by the annular flange, a plurality of reinforcing ribs that radially outward extend from the axis of rotation of the container housing.